NetFlow is an open but proprietary network protocol developed by Cisco Systems for collecting Internet Protocol (IP) traffic information. NetFlow runs on equipment with Cisco Internetwork Operating System (IOS), but served as the basis for RFC 2778. Cisco routers that have the NetFlow feature enabled generate NetFlow records that are exported from the router in User Datagram Protocol (UDP) or Stream Control Transmission Protocol (SCTP) packets and collected using a NetFlow collector.
Network flows can be been defined in many ways, but a common definition is as a 5-tuple, where a flow is defined as a unidirectional sequence of packets sharing the following values: (1) source IP address; (2) destination IP address; (3) source Transmission Control Protocol (TCP) port; (4) destination TCP port; and (5) IP protocol. The NetFlow protocol uses this definition.
Typically, a router outputs a flow record when the router determines that the flow is finished through, for example, the use of flow aging. Or a TCP session termination in a TCP flow might cause the router to expire the flow. Routers can also be configured to output a flow record at a fixed interval even if the flow is still ongoing.
A NetFlow record can contain a wide variety of information about the traffic in a given flow. For example, a Version 5 NetFlow record contains the following: (a) version number; (b) sequence number; (c) input and output interface for Simple Network Management Protocol (snmp) indices; (d) timestamps for the flow start and finish time; (e) number of bytes and packets observed in the flow; (f) Layer 3 headers, which include source and destination IP addresses, source and destination port numbers, IP protocol, and ToS (Type of Service) value; and (g) in the case of TCP flows, the union of all TCP flags observed over the life of the flow.
NetFlow records are usually sent via a UDP or SCTP in newer software and, for efficiency reasons, the router does not store NetFlow records once they are exported. The IP address of the NetFlow collector and the port upon which it is listening must be configured on the sending router. NetFlow is enabled on a per-interface basis in a router, which allows the router to export NetFlow records for dropped packets.
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